There are a multitude of filtration devices which separate a feed stock into filtrate and retained suspended matter which is too large to pass through the pore structures of the filter. A straight-through filter retains the suspended matter on the filter surface or within the filter matrix and passes only the filtrate. Cross-flow filters operate with tangential flow across the filter surface to sweep away suspended matter unable to pass through the filter surface pores. Cross-flow filters provide for the continuous extraction of retentate, or concentrated suspended matter, from one portion of the device and continuous extraction of filtrate from another portion.
Cross-flow filters can be constructed using multiple passageway, porous monoliths. Such monoliths can have tens of thousands of passageways extending through them, with the passageways normally parallel and uniformly spaced. When in use the feedstock is introduced under pressure at one end of the monolith, flows in parallel through the passageways, and a portion is withdrawn as retentate at the downstream end of the device, while a second portion passes through the membrane and porous monolith walls to exit at the periphery of the monolith.
Filtrate which passes into the porous monolith walls separating the passageways combines and flows through the walls toward the periphery of the monolith, and is removed through the outer skin of the monolith. The resistance to flow in the tortuous flow path of the monolith passageway walls can severely limit filtration capacity, and for this reason cross-flow filters based on large diameter high surface area, multiple passageway, porous monoliths are not found in wide commercial use.
Membrane devices utilize a semipermeable membrane to separate filtrate, also called permeate, from retentate. There is a multitude of different pressure driven membrane devices which separate and concentrate particles, colloids, macromolecules, and low molecular weight molecules. Membranes generally require a mechanical support which can be integral with the membrane, or separate. For example, membranes can be coated onto, or simply mechanically supported by, a porous support material.
Multiple-passageway, porous monoliths, e.g., honeycomb substrates, can be especially useful as membrane supports. In this instance membranes are applied to the passageway walls, which serve as both a mechanical support and as the flow path for filtrate removal to a filtrate collection zone. The walls of the substrate not only act as the supports for the membranes, but also serve as the egress path for the filtrate or permeate. In these pressure driven separation processes, the feed is forced through the small pores of the membranes that are supported on the walls of the honeycomb. Once the fluid has passed through the small pore size of the membranes (the path of most resistance), the filtered material enters the relatively larger pore of the walls. The amount of filtrate material that can pass from the innermost cell or cells, to the outside of the substrate, is limited by the wall porosity and thickness. If the walls are not sufficiently thick or sufficiently porous, the total volume of filtrate cannot be carried through the walls to the skin, and hence to the outside of the substrate. This results in a failure to use all of the available membrane surface area. That is, all or most of the feed that passes through the innermost cell or cells may simply pass through the support as retentate, without any portion passing through the membranes supported on those cells.